This invention relates generally to the art of depositing films and, more particularly, to an improved process and apparatus for photo-assisted chemical vapor deposition.
Thin silicon and germanium films have been formed by chemical vapor deposition (CVD) techniques which involve bringing SiH.sub.4 or GeH.sub.4, respectively, in contact with a hot substrate. Some of the gas molecules pyrolize, releasing Si and Ge atoms which remain on the surface and H.sub.2 molecules which are carried away by the gas stream. This pyrolysis reaction, like other reactions used in CVD, is an "activated" process. That is, it proceeds more slowly as temperatures are decreased. Substantial deposition rates can therefore be achieved with CVD only at temperatures on the order of 400 degrees Celsius or higher.
In many cases, it is desirable to minimize deposition temperatures. For example, some deposited films, particularly amorphous ones, are adversely affected by the temperatures required for conventional CVD. A substrate or device onto which a film is deposited can also be harmed by these temperatures.
Plasmas or glow discharges have been used to facilitate deposition at lower temperatures, with some success. However, the existence of the plasma places an entirely different set of constraints on a deposition environment, including substantial temperatures. In some cases, the ion bombardment which occurs in plasma deposition can be deleterious to film properties.
In a somewhat different context, ultraviolet (UV) radiation has been used to decompose gases by photolysis, causing deposition of elements from the gases. Examples of this work are found in A. Perkins et al., "The 147-nm Photolysis of Monosilane", Journal of the American Chemical Society, 101:5, 1979, and W. C. Krene "Photolysis of Diborane at 1849A", Journal of Chemical Physics, Vol. 37, No. 2, 1962, wherein UV radiation having a wavelength less than 2,000 angstroms is used to decompose SiH.sub.4 and B.sub.2 H.sub.6, respectively. However, UV radiation of less than 2,000 angstroms can be destructive to the constituent elements of many films, and to the deposited films themselves.
Therefore, in many applications it is desirable to deposit films at low temperatures, free of plasma-sustaining conditions and without subjecting the deposition gases or the films to energetic short wavelength radiation.